An engine obtains power by rotating a crankshaft with combustion of mixture gas in a combustion chamber. However, not all of an amount of mixture gas introduced to the combustion chamber is combusted. A portion of the mixture gas leaks into a crankcase via a gap present between a piston and a cylinder. This leaked gas is called blowby gas. It is legally prohibited, in particular in Japan, to discharge blowby gas, which is un-combusted gas, directly into the atmosphere as emission gas. For this reason, blowby gas is returned to an intake port via a PVC (Positive Crankcase Ventilation) channel to be mixed with newly introduced mixture gas for subsequent combustion together and eventual emission into the atmosphere.
Blowby gas contains therein a certain amount of lubricant oil such as engine oil in the form of oil mist. If blowby gas containing oil mist is returned to the intake port, the oil will adhere to the PCV channel and/or periphery of the intake port undesirably. Then, for the purpose of collecting oil mist present in blowby gas, an oil separator is provided inside a cylinder head cover or in midway of the PCV channel.
Patent Document 1 discloses an oil separator using a plurality of cyclones. With this oil separator, blowby gas entering via a gas introducing opening is introduced through a rectifying chamber to the multiple cyclones which are disposed side by side in series. Due to a centrifugal force by a swirling stream generated inside the cyclone, oil mist present in the blowby gas is flocculated and collected.